Aluminum alloy architectural sheet product and method for producing

ABSTRACT

IMPROVED ALUMINUM ALLOY SHEET CONTAINING SILICON, COPPER AND CHROMIUM IN CONTROLLED AMOUNTS AND HAVING A HIGH PURITY ALUMINUM BASE IS CAPABLE OF PRODUCING INTEGRALLY COLORED ANODIC COATINGS AND IS HIGHLY FORMABLE. THE INTEGRAL COATINGS CAN RANGE IN COLOR FROM GOLD THROUGH BRONZE SHADES TO A TRUE BLACK. THE ALLOY HAS IMPROVED CORROSION RESISTANCE AND IS HIGHLY SUITED FROM ARCHITECTURAL APPLICATIONS. THE PRODUCTION OF THE ALLOY SHEET PRODUCT INCLUDES A HIGH TEMPERATURE HOMOGENIZATION TOGETHER WITH CONTROLLED HOT AND WARM ROLLING PRACTICES TO ACHIEVE A FINE INTERNAL STRUCTURE.

United States Patent 3,827,952 ALUMINUM ALLOY ARCHITECTURAL SHEETPRODUCT AND METHOD FOR PRODUCING Edmund C. Franz, Pittsburgh, Pa.,assignor to Aluminum Company of America, Pittsburgh, Pa. No Drawing.Filed Sept. 20, 1972, Ser. No. 290,653 Int. Cl. C23b 9/02; C22f 1/04 US.Cl. 204-29 12 Claims ABSTRACT OF THE DISCLOSURE Improved aluminum alloysheet containing silicon, copper and chromium in controlled amounts andhaving a high purity aluminum base is capable of producing integrallycolored anodic coatings and is highly formable. The integral coatingscan range in color from gold through bronze shades to a true black. Thealloy has improved corrosion resistance and is highly suited for architectural applications. The production of the alloy sheet productincludes a high temperature homogenization together with controlled hotand warm rolling practices to achieve a fine internal structure.

Background of the Invention Aluminum sheet and other products have foundwide acceptance in the architectural field where they form substantialportions or even all of the exterior of several high-rise or other typebuildings. Especially useful are those aluminum products which, whenanodized, achieve an integrally colored oxide coating. The integralcolor is preferable to a dyed or otherwise imparted color since it ismore durable and has aesthetic advantages with respect to the depth ofthe color. However, aluminum alloy sheet products capable of economicaland rapid color development in anodizing baths together with goodforming characteristics and good corrosion resistance have beendifficult to achieve. For instance, some products can achieve a desiredcolor, for instance, a black color, but are not readily formed as bybending into the shapes desired for a given curtain wall design. Othersthat can be bent or otherwise formed tend to develop incipient cracks,folds or checking through the bend radius which disrupts the integrityof the anodic coating in this area. Moreover, the colored anodic coatingitself tends to exhibit a crackled or checked appearance in that theanodic coating appears much rougher through the radius of the bend asopposed to its appearance on fiat portions. This problem seems tiedsomehow to the hot rolling employed to reduce the thickness of the ingotor starting stock. 'However, economic production of a sheet productrequires a substantial amount of hot rolling using existing orconventional equipment. Another problem encountered with many aluminumalloy products in architectural applications is caused by atmosphericpollution which can corrode the sheet product even when anodized anddetract from its aesthetic qualities. For instance, certain atmosphericpollutants can cause an integral black color on an aluminum alloy sheetor plate product to exhibit a blue smudgy appearance which can beobjectionable.

DESCRIPTION In accordance with the invention an improved aluminumarchitectural sheet product is provided which has high resistance tocorrosion, is readily formed by bending or other working operations toproduce desired shapes which when anodized are substantially free fromany crackling or checking in areas where the sheet is bent. The sheet iscapable of producing an integral black color within 30 minutes employingexisting commercially accepted anodizing methods. The sheet is alsocapable of achieving gold or other colors by altering the anodizingconditions.

The improved sheet is fashioned from an improved alloy consistingessentially of 0.4 to 0.6% copper, 0.1 to 0.25% chromium, 0.08 to 0.18%silicon, the balance being aluminum and not more than the followingamounts of other elements and impurities: iron 0.3%, manganese 0.05%,zinc 0.05%, titanium 0.05%, magnesium 0.1%, all others being limited to0.05% each and a total of 0.1%. Further the total amount of allimpurities not including iron, should not exceed 0.2%. While magnesiumhas been described as an impurity, one preferred embodiment of theinvention contemplates controlled additions of magnesium within therange of up to 0.1%, the preferred minium for magnesium being 0.005% andthe preferred maximum being 0.05%. Within these limits magnesium confersan advantage on the improved sheet with respect to reducing any tendencyfor structural streaking.

The improved method for producing the architectural sheet productcontemplates providing a body of the alloy described above andhomogenizing it at a sufficiently high temperature to dissolve thechromium and copper containing phases after which the metal is hot-warmrolled at controlled temperature levels to produce a sheet product. Thesheet product so produced can, if desired, be further worked by coldrolling which can advantageously strengthen the sheet by strainhardening and the cold rolling can be preceded by an anneal. If ease offorming is important any cold rolling can be followed by an anmeal.

The alloy is produced as an ingot by any of the means conventionallyemployed although continuous casting is preferred. Economical productionrequires starting with an ingot of substantial thickness, at least 8inches and preferably 10 inches or more in thickness, for instance 12inches or even greater. However, the hot rolling normally required infabricating a rolled product from starting stock of this size seems tocause the bend appearance problem discussed above regardless ofsubsequent operations or treatments. However, as explained below certainspecial controls in accordance with the present improvement alleviatesthis condition thereby enabling economical production of an improvedarchitectural sheet product.

The ingot is homogenized by heating to a temperature of 1000 to 1 F.,preferably 1050" to 1100 F., for a period typically ranging from about 4to 20 hours or more. This assures solution of the copper and chromiumcontaining phases and further spherodizes the constituent phases such asAl-Fe-Si and Al-iFe-Cr constituents. The ingot is then cooled to roomtemperature or cooled to about 800 F. for rolling. It is preferred thatany cooling be fairly rapid as by cooling in open .air as opposed tofurnace cooling and the improved method favors such rapid cooling ratesas are commensurate with open air cooling.

The homogenized ingot is scalped and rolled at controlled temperaturesranging from 300" to 800 F. In accordance with the invention it has beenfound that the initial hot rolling passes impart certain characteristicsto the grain structure of the rolling stock which are reflected in thefinal product regardless of the subsequent fabrication steps even ifthey include drastic cold reductions. It has been found that theimproved sheet alloy described above cannot sustain working operationsabove temperatures of about 850 F. without exhibiting appearanceproblems in bend areas. This effect seems connected with the fact thatconventional hot working operations at temperatures of 850 to 950 F. canelongate the grains considerably without any significant amount of grainfragmentation occurring. While the grain elongation effect is not ofspecial significance it is especially important that the very firstpasses produce at least some significant amount of grain fragmentationacross the long transverse direction or dimension as it is this whichavoids the development of any banded grain structure. The long transverse direction is that corresponding to the sheet width, that is normalto the sheet length and to the sheet thickness. -It has been found thatthe first hot rolling passes set the pattern for this effect whichpattern exerts a strong influence on the final product regardless ofsubsequent fabricating operations, even drastic cold rolling and thedrastic grain fragmentation associated therewith. In accordance with theinvention it is important that rolling operations at temperatures ofover 800 be substantially avoided since they produce grain elongationwithout sufficient grain fragmentation along the long transversedimension. Accordingly, the invention contemplates avoiding suchoperations at temperatures over 800 F. and instead contemplates hotrolling at rolling mill entrance temperatures of 600 to 800 F.,preferably 700 to 800 -F. That is, the ingot or rolling stock is cooledto or reheated to provide a substantially uniform metal temperature of600 or 700 to 800 F. as the metal enters the hot rolling mill. The metalcan, however, be brought to a higher temperature, say 825 or 850 or even900 -F. prior to rolling, so long as it is cooled to substantially notover 800 F. as it enters the rolling operations.

The invention contemplates substantial reductions at elevated butcontrolled temperatures which can economically be carried out in hot andcontinuous mills. Accordingly, the rolling stock is hot warm rolled attemperatures between the entrance temperature of not over 800 F. down totemperatures of 300 F. or higher. It is preferred that temperaturesduring the hot-warm rollingdo not fall below 375 F. and preferably notbelow 450 F.

The extent of the rolling reductions at the controlled elevatedtemperatures should amount to at least 40% of the thickness of thestarting stock and preferably at least 80%, and it is preferred that thereductions proceed without reheating the metal to compensate for heatlost in the rolling operation. These reductions at these controlledtemperatures result in a grain structure which, while some- Whatelongated, is well fragmented in the transverse direction, the widestgrain or grain fragment dimension being not over about 0.05 millimeteras measured in the transverse direction of the sheet at the conclusionof the controlled temperature rolling operations.

It has been found that the above-described alloy composition andhot-warm controlled rolling procedures which produce the describedfragmented grain structrue combine to produce a highly unique product,an economical architectural sheet capable of producing a rapidlydeveloped integral anodic black color in existing anodizing equipmentand procedures which sheet product can be bent without exhibitingincipient cracks, checking, kinks or any generally rough appearingsurface condition thereby enabling the production of bent sheet shapesfor architectural applications free of these defects which plaguedprevious products capable of approaching this type of color development.Furthermore, it has been found that the improved sheet product exhibitssubstantially complete freedom from structural streaking and, verysignificantly, greatly increased resistance to corrosion in pollutedatmosphere thus providing for greatly improved and reduced maintenancecosts on a completed building, especially a high-rise building whichextends into the polluted atmospheres which seem to lurk above almostevery site where large buildings are constructed.

The sheet product exiting the controlled warm rolling operation cantypically range in thickness from 0.075 to 0.25 inches and is useful inthese and other thickness with or without further processing. Ifdesired, the sheet can be annealed in order to soften it for furtherworking or forming operations. For instance, it can be fully annealed ata temperature of around 600 to 700 F. and the sheet s0 treated exhibitsless tensile strength but is easier to bend than in the condition as itexits the warm rolling operation. If desired the sheet product can befurther worked as by cold rolling with or Without tfirst annealing. Thestrain hardening effects of cold rolling would confer higher strength tothe sheet which may be desired in some instances. Additionally someamount of cold rolling can sometimes reduce any structural streakingproblems in the anodized sheet product. The desirability of annealingbefore cold rolling would depend to some extent on the degree of thecold reduction.

One preferred embodiment of the invention contemplates providing thedefined improved alloy as a cladding layer on an alloy core to provide acomposite structure. This has the advantage of achieving a desiredstrength level from the core material while realizing the advantages ofthe invention in the cladding layer. A suitable core alloy is alloy 3003which contains nominally 1.2% manganese and 0.12% copper, balanceessentially aluminum. Thus the invention contemplates the use of suchclad composites featuring the improved alloy as a cladding on one orboth faces of a sheet produced and a core composed of an aluminum alloywhich aluminum alloy may typically contain up to 2%, for instance 0.3 to2% of manganese. This type of core alloy is highly compatible with theimproved alloy with respect to rolling and other fabricatingcharacteristics thus enabling relatively economical production of a cladproduct. When fabricating such a composite the cladding material in theimproved alloy is fabricated into sheet according to the mehods heredescribed. This sheet is placed on one or both sides of the core rollingstock and hot roll bonded thereto to provide a bonded composite which isreduced by rolling. The roll ing here does not need to proceed accordingto the improved controlled conditions since the cladding has alreadybeen previously broken down by the improved method.

The sheet product can be anodized to develop integrally colored anodiccoatings. Employing an aqueous bath containing 130 to 200 grams perliter of sulfuric acid at a temperature of 70 to F. and current densitylevels of 20 to 30 amps per square foot and maintaining anodizingconditions for 20 to 60 minutes a coating is produced ranging inthickness from about 0.7 to 1.2 mils and the coating exhibits anintegral gold or yellow color. Using an aqueous bath containing aboutgrams of sulfophthalic acid and 5 grams of sulfuric acid per liter andmaintained at temperatures of about 70 or 80 F. and current densitylevels of 25 to 30 amps per square foot a black integral anodic coatingcolor is achieved within about 30 minutes. The black color ischaracterized by apparent reflectance and yellowness levels of less than4% as determined by Color-Eye measurement. Again the coating is about0.71 mil in thickness. Instead of sulfophthalic acid the electrolytecould contain one of the other sulfonic acids such as sulfosuccinic andsulfosalicyclic acids.

It is usually desirable to seal the anodic coating, for example, byimmersing in hot (210 F.) water or other suitable solutions. Thecoloration and texture developed in the anodic treatment can be modifiedby treatment of sheet prior toanodic oxidation. The surface can bechemically brightened by washing with a solution of phosphoric andnitric acids or electrochemical procedures. Mechanical treatments suchas buffing, polishing, sand blasting and the like can also be employedto alter the texture of the surface prior to anodizing.

As indicated earlier the improved architectural sheet product whenanodized exhibits improved resistance to corrosion thus rendering ithighly suited for high-rise and other buildings such as in cities wherepolluted atomspheres may be present. In comparing different anodizedsheet products an accelerated test has been developed which serves toindicate potential susceptibility to corrosion in highly pollutedenvironments. Since the corrosion occurs through the anodic coating andis dependent on weaknesses in the coating the test is directed to thecoating. In this test, sample panels anodized under conditions describedpreviously for developing integral-black coatings on the subject alloyof this invention are immersed in a solution of 200 grams CuSO and ml. H80 in 1 liter of water. A cathodic potential of 1.5 volts is applied forthree minutes. By this procedure copper is deposited in any voids orholes in the anodic coating. In this test the improved sheet showedvirtually no copper deposition which is indicative of a sound anodiccoating of very high integrity and thus indicative of very goodresistance to the corrosion elfects of polluted atmospheres. On theother hand, panels of a commercial architectural sheet alloy, 6061 alloycontaining nominally Al, 1% Mg, 0.6% Si, 0.25%, Cu and 0.2% Cr, used forintegral color anodizing exhibited numerous spots of Cu depositionindicative of weak points in the coating thus leaving the anodized sheetquite susceptible to corrosion by atmospheric pollutants.

Example 1 A sheet product was fabricated from an alloy containing 0.47%Cu, 0.18% Fe, 0.07% Si, 0.17% Cr, 0.00% Mg, 0.02% Zn, 0.01% Ti, thebalance aluminum. The alloy was continuously cast as an ingot which washomogenized at a temperature of 1100 F., cooled to room temperature inopen air, scalped and then reheated to about 950 F. The scalped ingotwas then introduced into a hot reversing mill at a conventional ingotmetal temperature of 910 F. which reduced the thickness from about 12inches to about 4 inches, a reduction of 67%. The 4-inch plate was thenintroduced to a second mill which reduced its thickness to about inch.This plate was then run through a continuous mill at an entrytemperature of about 650 F. which reduced its thickness to about A; inchat an exit temperature of about 350 F. Sections of the sheet so producedwere bent over a 4T radius (a radius of 4 times the sheet thickness)along a longitudinal direction and exhibited surface cracking folds, andobjectional kinking and checking along the bend radius. The bent sheetsections were anodized in the sulfophthalic-sulfuric acid aqueousanodizing bath described above for 30 minutes to develop an integralblack coating. Metallographic examinations made of transverse sectionsthrough the bend areas revealed objectionable thinning of the anodiccoating in localized areas as a consequence of irregular formation ofthe coating over the roughened surface which considerably diminished theintegrity of the coating. The anodized bend area also exhibited visualdefects including kinking and surface cracking or checking which,regardless of coating integrity, themselves are highly objectionable forarchitectural application.

Example 2 Improved sheet product was fabricated from an alloy containing0.52% Cu, 0.22% Fe, 0.14% Si, 0.06% Mn, 0.007% Mg, 0.16% Cr, 0.025% Ti.The alloy was continuously cast as an ingot which was homogenized at atemperature of 1100 F., cooled .to room temperature in open air, scalpedand then reheated to a temperature of 800 F. The scalped ingot was thenintroduced into a hot reversing mill at an ingot metal temperature of760- 7-80 F. which reduced the thickness from about 14 inches to about 6inches, a reduction of over 55%. The temperature exiting the mill wasabout 720 F. The 6-inch plate was then fed to a second mill whichreduced its thickness to about 1 inch and the exit temperature of thesecond mill was about 650 F. The total hot reductions to this pointamount to over 90% of the 14 inch thick starting stock. The l-inch platewas then run through a hot continuous mill where it sustained an 80%reduction to produce a sheet approximately /a inch in thickness at anexit temperature of about 350 F. Sections of this sheet were anodized inthe sulfophthalic-sulfuric acid aqueous anodizing bath described abovefor 30 minutes and the integral coating exhibited a jet black colorcharacterized by apparent reflectance and yellowness values both under4% as determined with the Color-Eye instrument. Sections of this sheetalso were bent over a IT radius along a longitudinal direction. Thisbend is four times more sharp, and hence more severe, than thatmentioned in Example 1. Yet the specimens exhibited no incipient cracks,folds or objectionable checking along the bend radius. The bent sheetsections were anodizied using the above-described conditions to developthe jet black color. Metallographic examinations made of transversesections through the bend areas confirmed that the anodic coating was ofvery high integrity and further exhibited no visual defects.

What is claimed is.

1. A method of producing improved architectural sheet comprising thesteps:

(1) providing a body of aluminum base alloy consisting essentially of0.4 to 0.6% copper, 0.1 to 0.25 chromium, 0.08 to 0.18% silicon, thebalance aluminum and not more than the following amounts of otherelements and impurities: iron 0.3%, manganese 0.05%, magnesium 0.1%,zinc 0.05%, titanium 0.05 all other elements and impurities beinglimited to 0.05% each and a total of 0.1%, the amount of all elementsother than aluminum, silicon, copper, chromium and iron not exceeding0.2%.

(2) homogenizing said body by heating it to a temperature within therange 1000 to 1150 F.,

(3) hot rolling said body at temperatures which are controlled such thatthe metal temperature as it initially enters the rolling operation isbetween 600 and 800 F. and substantially avoiding metal temperatures inexcess of 800 F. during rolling to produce a rolled product having agrain structure featuring a substantial amount of grain fragmentationacross the long transverse direction.

2. A method of producing improved architectural sheet comprising thesteps:

(1) providing a body of aluminum base alloy consisting essentially of0.4 to 0.6% copper, 0.1 to 0.25% chromium, 0.08 to 0.18% silicon, thebalance aluminum and not more than the following amounts of otherelements and impurities: iron 0.3%, manganese 0.05%, magnesium 0.1%,zinc 0.05%, titanium 0.05 all other elements and impurities beinglimited to 0.05% each and a total of 0.1%, the amount of all elementsother than aluminum, silicon, copper, chromium and iron not exceeding0.2%.

(2) homogenizing said body by heating it to a temperature within therange 1000 to 1150 F.,

(3) rolling said body to reduce its thickness and alter its grainstructure such that a substantial degree of grain fragmenation occurs inthe long transverse direction, said fragmentation being initiated in thefirst rolling passes by avoiding in these passes metal temperatures inexcess of 800 F., the body entering said initial roll passes at a metaltemperature of between 600 and 800 F., to produce a rolled producthaving a fragmented grain structure such that the widest grain or grainfragment dimension in the long transverse direction does not exceedabout 0.05 millimeter.

3. A method of producing improved architectural sheet comprising thesteps:

(1) providing a continuously cast body of aluminum base alloy consistingessentially of 0.4 to 0.6% copper, 0.1 to 0.25% chromium, 0.08 to 0.18%silicon, the balance aluminum and not more than the following amounts ofother elements and impurities: iron 0.3%, manganese 0.05%, magnesium0.1%, zinc 0.05 titanium 0.05%, all other elements and impurities beinglimited to 0.05 each and a total of 0.1%, the amount of all elementsother than alurninum, silicon, copper, chromium and iron not exceeding0.2%,

(2) homogenizing said body by heating it to a temperature within therange 1000" to 1150 F.,

(3) cooling said body at a cooling rate commensurate with open aircooling, and

(4) hot-warm rolling said body to efiect a reduction in its thickness ofat least 40% at controlled temperatures, said rolling reducing thethickness and altering the cast internal structure of said body suchthat a substantial degree of grain fragmentation occurs in the longtransverse direction, said fragmentation being initiated in the firstrolling passes by avoiding in these passes metal temperatures in excessof 800 F. the body entering said initial roll passes at a metaltemperature of between 600 and 800 F., said rolling proceeding withoutreheating the metal body, the metal temperature during said rollingdecreasing but to a level not below 300 F., said rolling producing arolled product having a fragmented grain structure such that the widestgrain or grain fragment dimension in the long transverse direction doesnot exceed about 0.05 millimeter.

4. The method according to claim 2 wherein said rolled product isfurther reduced in thickness by cold rolling.

5. The method according to claim 1 wherein said alloy contains acontrolled amount of magnesium of up to 0.1%.

6. The method according to claim 5 wherein the magnesium ranges from0.005 to 0.05%.

7. The method according to claim 3 wherein the hotwarm reduction in saidstep (4) amounts to at least 80%.

8. The method according to claim 1 with the additional step of anodizingsaid rolled product to produce an integral colored anodic coating.

9. An improved architectural sheet product composed of an alloyconsisting essentially of 0.4 to 0.6% copper, 0.1 to 0.25% chromium,0.08 to 0.18% silicon, the balance aluminum and not more than thefollowing amounts of other elements and impurities: iron 0.3%, manganese0.05%, magnesium 0.1%, zinc 0.05%, titanium 0.05%, all other impuritiesbeing limited to 0.05% each and a total of 0.1% the amount of all theseelements and impurities other than aluminum, silicon, copper, chromiumand iron not exceeding 0.2%, the sheet product exhibiting a substantialdegree of grain fragmentation in the long transverse direction such thatthe largest grain or grain fragment size does not exceed about 0.05millimeter in the long transverse direction as a result of controlledhot-warm rolling operations at temperatures of 300 or more but not toexceed 800 F., said sheet when bent to a radius of one time itsthickness exhibiting substantial freedom from checks in the bend radius,said sheet further being characterized when anodized by the ability todevelop an integral true black color said anodic coating having animproved level of resistance to attack by polluted atmospheres.

10. The improved sheet product according to claim. 9 wherein said alloycontains a controlled amount of magnesium of up to 0.1%.

11. The improved sheet product according to claim 10 wherein themagnesium ranges from 0.005 to 0.05%.

12. The improved sheet product according to claim 9 wherein the sheetproduct features at least one bend of a radius one time its thickness ormore and features an integrally bonded anodic coating having an integralblack color characterized by maxima of 4% for yellowness and apparentreflectance.

References Cited UNITED STATES PATENTS 1,913,846 6/1933 McCullough 1432,941,930 6/1960 Mostovych et a1. 148-115 A 3,113,002 12/1963Hollingsworth 148--31.5 3,164,494- 1/1965 English 148-315 3,187,4286/1965 English l48-11.5 A 3,370,943 2/1968 Beatty 75143 WAYLAND W.STALLARD, Primary Examiner U.S. Cl. X.R.

Patent No. 3,827,952 Dated August 6, 1974 Inventofl) Edmund C. Franz Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Col. 3, line 64 Change "atmosphere" to atmospheres-- Col. 4, line 23Change 'produced" to -product-- Col. 5, line 68 Change "65 to --650--Signed and sealed this 12th day of November 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. 6 C. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PO-105O (10-69) USCOMM-DC 6O376-P69 U. 5. GOVERNMENTPRINTING OFFICE I969 0-366-334,

